Monolithically integrated clock and data recover (CDR) circuits often use ring oscillators as voltage controlled oscillators (VCOs). Ring oscillators, especially in complimentary metal oxide semiconductor (CMOS) technology, exhibit large frequency variations with process, voltage, and temperature variations. Therefore, these VCOs need a wide tuning range even for operation at a single desired frequency. Furthermore, if a phase-locked loop (PLL) is intended to be used over a wide range of input frequencies, then even a VCO with a small amount of process variation will need a wide tuning range. Providing a wide tuning range typically requires a large tuning gain over a limited tuning voltage range. However, the larger the tuning gain, the more sensitive the VCO is to noise on the tuning signal. Sensitivity to noise can be a major problem in an integrated circuit that has several CDR channels or a large digital section in addition to the CDR.
One technique that is used to reduce the magnitude of the required tuning gain and to reduce the corresponding noise sensitivity while still providing a wide frequency range involves splitting the entire frequency tuning range of a VCO into multiple overlapping frequency bands. VCOs that are configured with multiple overlapping frequency bands are generally referred to as “multi-band VCOs.” Controlling multi-band VCOs to lock onto the setpoint frequency of the VCO is a two step process. The first step involves “centering” the VCO by selecting a frequency band that includes the setpoint frequency. In most multi-band VCO implementations, the frequency bands overlap each other at their boundaries and therefore centering involves selecting the frequency band with the center frequency that is nearest to the setpoint frequency. Centering can be achieved by a digital switching mechanism that is less sensitive to noise than single-band VCOs that rely on a large tuning gain to span the entire frequency tuning range. Once the frequency of the VCO is centered near the setpoint frequency, the second step involves adjusting the analog phase-locked loop (PLL) control voltage to drive the VCO to phase and frequency lock.
Different techniques have been developed to carry out the above-identified centering function. These techniques include centering only at power-up or reset, centering with an all digital VCO control, and centering using a secondary analog control loop. Although these techniques work well, there is still a need for an improved technique for centering a multi-band VCO.